Conventionally, a waste heat recovery device is known for recovering waste heat from supercharging air supplied to an engine. For example, Japanese Unexamined Patent Publication No. 2013-167241 discloses a waste heat recovery device, including: a heat exchanger which evaporates an working medium by exchanging heat between pressurized intake air and an working medium; an expander which expands the working medium which has flowed out from the heat exchanger; a power generator connected to the expander; a condenser which condenses the working medium which has flowed out from the expander; a pump which sends the working medium which has flowed out from the condenser, to the heat exchanger; and a controller which controls the number of revolutions of the pump in accordance with the thermal load on the heat exchanger. When the thermal load becomes larger, the controller increases the number of revolutions of the pump on the basis of previously stored information. Consequently, the amount of waste heat recovered (the amount of electric power generated by the power generator) is increased. On the other hand, when the thermal load becomes smaller, the number of revolutions of the pump is reduced on the basis of the information. In other words, the vaporization of the working medium in the heat exchanger is prioritized.
In the waste heat recovery device disclosed in Japanese Unexamined Patent Publication No. 2013-167241, it is difficult to recover waste heat stably in a broad range from a low to a high thermal load state of the heat exchanger (prime mover load state). More specifically, when the thermal load on the heat exchanger declines, the amount of heat which can be recovered by the working medium in the heat exchanger is extremely small, and therefore even if the controller reduces the number of revolutions of the pumps, there is a risk of the working medium flowing out from the heat exchanger and flowing into the expander, in a gas-liquid two-phase state where the working medium is not sufficiently evaporated in the heat exchanger. In order to prevent a situation of this kind, it is necessary to stop the pump (stop the recovery of waste heat), when the thermal load has declined. In other words, in the waste heat recovery device disclosed in Japanese Unexamined Patent Publication No. 2013-167241, it is not possible to recover the waste heat stably, especially in a state of low thermal load.